The above-referenced copending application discusses observed differences between loudspeaker constructions utilizing non-conductive ceramic magnets and loudspeaker constructions utilizing alnico-type magnets and points out the general inferiority of the ceramic magnet structures. In particular, the effects of distortion due to eddy currents, in the top and bottom plates, which generate local magnetic fields that are coupled back to the voice coil are noted. These eddy currents produce harmonic distortion effects due to the non-linear iron characteristics as well as frequency selective, i.e., frequency dependent, distortion effects because the amplitudes of the eddy currents are proportional to frequency.
In the prior art, the effects of energy loss due to eddy currents in the conductive parts of an electroacoustic magnetic transducer motor structure have been misunderstood. For example, extra conductive material, generally in the form of copper, has been added to the motor structure to flatten the loudspeaker impedance characteristic, i.e., make the characteristic more uniform with frequency. The fact that the energy transferred into the conductive material reduces the energy that is transformed into useful loudspeaker diaphragm motion, and that this effect, which is non-uniform with frequency, results in a reduction in the accuracy of reproduction of transients, has either not previously been recognized or has been ignored. This frequency selective loss in prior art transducer constructions results in transducers with reduced ability to track the rapid changes in audio signals. Indeed, a flat impedance characteristic in a loudspeaker driver has been found to be of secondary importance and is even undesirable when it is produced by non-linear or frequency selective losses in any of the parts of the magnet structure.
It has been discovered that in addition to eddy current effects in ferromagnetic structure parts, ceramic magnets in contrast to alnico magnets, introduce another distortion component. Magnetic fields are introduced into the magnet material by the motion of the signal-carrying coil, whether in a loudspeaker or a microphone embodiment. This energy, which is effectively subtracted from the available useful energy, is proportional to coil travel and is thus inversely proportional to frequency. There are undesirable consequences associated with the phenomenon. For example, the signal-related AC magnetic energy that is induced into the magnet causes distortion. While the exact mechanism has not yet been proven, it is believed that the induced AC magnetic field modulates the DC field in the magnet.
The prior art also recognized the need for improved low frequency reproduction, but in failing to recognize the role of magnetic and eddy current losses as discussed above, attempted solutions that were incorrect and expensive. Attempts to boost low frequency output are exemplified in U.S. Pat. No. 3,838,216 to Watkins and U.S. Pat. No. 4,504,704 to Takashi et al. In these patents, a second voice coil and large value inductors and capacitors are used to form a frequency selective bass boost circuit. Besides the considerable expense of the extra parts and the construction, these circuits suffer from time displacement distortion due to the long time constants of the boost circuits. This results in bass reproduction that may be powerful, but is slow and inaccurate.
In the present invention, powerful, yet quick, accurate bass reproduction is achieved by the simple, inexpensive technique of slotting the ceramic magnet. A slotted ceramic magnet seems to function as a stabilizing means for reducing distortion caused by the signal related magnetic fields induced into the magnet. In a further aspect of the invention, it has been found that combining the slotted or split magnet construction with the split plate technology described in the above-mentioned copending application, produces results that surpass the expected combined contributions and represents a very attractive electroacoustic magnetic transducer construction.
In a still further aspect of the invention, a slotted "bucking" magnet and an additional slotted back plate, when added to both a conventional transducer and to a transducer incorporating a slotted magnet, produces very beneficial results.